Wohl-Ziegler Reaction

The Bromination Of Allylic Positions With N-bromosuccinimide (NBS) Follows A Radical Pathway.

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Mechanism Of The Wohl-Ziegler Reaction

It Is Very Important To Keep The Concentration Of Br₂ and HBr Low To Prevent Side Reactions Derived From Simple Ionic Addition With The Alkene. These Reagents Are Therefore Generated In Situ From NBS. The Catalytically Active Species Is Br₂, Which Is Almost Always Present In NBS Samples (red Colour).

A Radical Initiator (UV, AIBN) Is Needed For The Homolytic Bond Cleavage Of Br₂ :

The Allylic Position Is Favoured For Hydrogen Abstraction, Because The Resulting Radical Intermediate Is Resonance Stabilized: 

Regeneration Of Br₂:

Bromination:

Bromination Is Favored To Occur At The More Highly Substituted Position, Because The Corresponding Intermediate Radicals Are Better Stabilized.

CCl₄ is The Solvent Of Choice, Because NBS Is Poorly Soluble And Resulting Succinimide Is Insoluble And Floats At The Surface. This Keeps The Concentration Of Reagents Low And Is A Signal That The Reaction Is Finished.

However, Environmental Concerns Have All But Eliminated The Use Of CCl₄, And Its Replacement, CH₂Cl₂, Is Being Restricted As Well. Many Other Solvents Are Reactive Toward NBS, And Are Thus Unsuitable, But Acetonitrile Can Be Used To Good Effect.

Recent Literature

A One-Pot, Transition-Metal-Free Procedure For C-O, C-S, And C-N Bond Formation At The Benzylic Position Of Methylarenes
H. Shimojo, K. Moriyama, H. Togo, Synthesis, 2015, 47, 1280-1290.